The current submission is a competitive revision application in response to NOT-OD-09-058 (NIH Announces the Availability of Recovery Act Funds for Competitive Revision Applications) and serves as a supplement to grant RO1 NS028829 aimed at uncovering the mechanism by which MEF2 transcription factors regulate synapse number during development by activating a program of gene expression that controls activity-dependent synaptic remodeling. We have made unanticipated progress in our studies of MEF2 targets and would like to capitalize upon this progress during the next years by recruiting additional personnel to this project. Our initial characterization of the transcriptional program induced by MEF2 in hippocampal neurons (Specific Aim 3 of our original proposal) indicated that the E3 ubiquitin ligase Ube3A is a direct transcriptional target of MEF2. Ube3A is of particular interest because loss-of-function mutations in this gene in humans cause Angelman syndrome (AS), a debilitating neurodevelopmental disorder that affects approximately one in 15,000 individuals. An important goal of Aim 3 in our original application was to test the hypothesis that the induction of Ube3a transcription contributes to MEF2-mediated synaptic remodeling. In the course of these studies we have found that Ube3A regulates excitatory synapse development by controlling the degradation of Arc/Arg3.1, a synaptic protein that promotes the internalization of AMPA-type glutamate receptors. Disruption of Ube3A function in neurons leads to an increase in Arc/Arg3.1 expression and a concomitant decrease in cell surface AMPA receptors. Importantly, this decrease in surface AMPA receptor expression, and an accompanying increase in the number of synapses that form or are maintained on Ube3A-deficient neurons, can be rescued by Arc/Arg3.1 knockdown. These findings have significant implications for the AS etiology, suggesting that excess Arc/Arg3.1 expression may contribute to at least some aspects of the cognitive dysfunction associated with AS. This supplemental proposal therefore seeks to expand the scope of the original application to test the hypothesis that elevated Arc/Arg3.1 levels contribute to AS-associated synaptic and behavioral phenotypes in Ube3A-deficient mice. To this end, Ube3A-deficient mice will be crossed with Arc/Arg3.1 knockout animals to determine if loss of a single copy of the Arc/Arg3.1 locus acts to ameliorate the Ube3A loss-of-function phenotypes. It is our hope that the proposed experiments will provide a better understanding of Ube3A function, give insight into the mechanisms by which activity-dependent gene expression acts to modulate synaptic function, and provide new opportunities for the development of therapeutic strategies to alleviate AS pathology. PUBLIC HEALTH RELEVANCE: Angelman syndrome is a debilitating disorder of cognitive function that affects approximately one in 15,000 individuals. Building on our preliminary findings, the proposed study will test the hypothesis that elevated levels of a specific protein in the brain contribute to the cellular and cognitive deficits observed in this disorder.